When I get a chance, I will have to give these mods a try. I suspect changing C301 from a .047uf to a 10uf will improve the low frequency response. Ed

Correct it improves the bass, may be too much. My initial results were not as good as John's. I elected to cut the two caps out and use the leads to tack the new ones on. For the (C301) I tried: 10 uF, 4.7 uF, 2.2 uF, 0.5 uF and 0.04 7uF, first two electrolytic, last three unpolarized. The negative of the polorized caps were further away from self tuning unit.

The (C337) I replaced with 1000 pF, 100 pF, 68 pF and 39 pF. I did not have a 47 pF handy.

I started high with 10 uF and 1000 pF and worked my way down towards the original values, which I ended at. I won't get into the many iterations, but there were many. The sound was just way distorted, bass wise. I have it back sounding fine again with near original values. The caps in it now are high end film and mylar, so it sound better. It was sounding fine before but I think just better caps help. To originals got sacrificed cutting them out so I can't put them back in.

It seems like for me something between 0.50 and 0.05 uF will be best for C301, which is not what I posted. Tomorrow I will try the 1000pF and 100pf at C337, see if I can narrow that down. I don't have a 470pF.

C301 - 0.047 uF to 0.547 uF. I found with the 0.047 uF soldered in
and touching a 0.10 uF to it, for a total of 0.147 uF was ideal. May
between 0.147 uF to .500 uF is a sweep spot. I don't want to
finalize it until I put the input mod on. Why mine need a smaller
cap by about a factor of 20 or more I don't know. I rechecked 2.2
uF and it sounded bad.

C337 - is originally 47 pF but measured 62 pF. Going to a 39 pf
made treble brighter, which I like. Just clipping on 10 pF dulled the
treble. A 68 pf to the 39 pF made real dull highs. So I am glad I
clipped the original out. You may leave that one 47 pF. The mod I
was given called for 470 pF to 1800 pF. That seems to be too
much. May be C307 and C337 interact. Still tiny decrease here
seems to be goodness. I am pleased.

Before I finalize this I want to make the modification to the input
as shown. That should be straightforward and improve sound. I
also played with the external antenna like John did. I lost range.
Also the external antenna changes the sound slightly. I had to
back off the bass on the EQ from the source.

This is my high tech TH installation. Seems to work well and the audio quality is as good or better than the best AM BCB stations in my area. The audio source is PC with my MP3's or Internet radio, Pandora Radio or SHOUTcast (WinAmp). The sound shaping program is Winamps EQ and Stereo Tools, a plug-in to WinAmp. This provides compression and limits and other fancy stuff. The sound with Stereo Tools off is still amazing. The limits and compression just allows you to run to the limits and keep from exceeding the audio input of the TH, for various music dynamics from soft to loud music.

The wires, power, gnd, antenna (L to R) and audio (bottom) are all separated.
(Note: the RF approved cardboard box, have to make a shelf)

I want to make a base loaded or mid loaded vertical antenna and plant that in the yard away from the house for fun.

PS There is a Patent for this with the schematics (not with detailed values, but enough to explain the circuits). Google:

Very interesting guys! I have 3 of these transmitters, 2 ver 5 and 1 ver 4.6. - I don't seem to have any of these troubles with my units regarding bad audio, they all sound really great.
My question is, do you know for sure where the transmitter outputs are (transistors) located?. And could you give me a hint which replacement transistors would increase the power output. I have plenty of IRF640N Mfets here which I used in AM Amps.
Before anyone tells me 100mW is the legal power, I live in Scotland, so FCC doesn't matter here. The units can't be licensed in my country at 100mW anyway, so nothing to lose. I am interested in experimenting. I don't see any harm in up-ing the output a bit 500mW or 1W nothing extreme. There is other ways, buy an AM amplifier pallet from eBay and add it into the box with a heat sink. But I am again only looking at 1W max. There must be something I can change that will let more RF out.

Very interesting guys! I have 3 of these transmitters, 2 ver 5 and 1 ver 4.6. - I don't seem to have any of these troubles with my units regarding bad audio, they all sound really great.My question is, do you know for sure where the transmitter outputs are (transistors) located?. And could you give me a hint which replacement transistors would increase the power output.

This has been discussed on other forums by people who know way more than I do. It is not possible to increase the power with just a swap of transistors with out a major redesign. Why? Not 100% sure. My guess is the power supply. You (again guessing) have to change the voltage regulation and increase the available voltage. With that said I have had my TH on the scope and it looks like it is making way more power (especially on the lower Freqs) than 100 mW already. I can't say for sure because I am using an oscilloscope with a 50 ohm load, not a proper calibrated milli-watt meter. I have tried a few tests, comparison to a calibrated signal Gen. I think (guessing again) it might be 140 mW off the external antenna F-jack (not the indoor antenna terminal). The regs in the US is actually 100 mW to the antenna. So 40 mW of loss in 100 feet of coaxial would be not be usual. It's all on the up and up since the unit is FCC Part 15 approved. So I have to assume they know what they are doing. Again my measurements may be off.

I have talked to a Gent in the Netherlands. The topic of TH modifications came up, including RF power increase. He told me he looked into it, and he abandoned the idea. The circuit board and design is just not easy to modify is what I gather. However I'm not saying you can't try, but plan on changing the power supply. This will affect the PLL circuit, which for some screwed up design reason is on the same power as the RF (which is bad form). Separating them would be a good thing as a side benefit, stabilizing the PLL, which has some "wobble" or FM'ing already. Bottom line, not having tried my self, only hearing this second hand, it sounds like more than a simple transistor swap.

Quote:

I am interested in experimenting. I don't see any harm in up-ing the output a bit 500mW or 1W nothing extreme. There is other ways, buy an AM amplifier pallet from eBay and add it into the box with a heat sink. But I am again only looking at 1W max. There must be something I can change that will let more RF out. Thanks for any help!

If you want to experiment I would go with your plan B, get a kit, like that [[ AM Stereo / CQUAM kit ]] on ebay (also youtube series of video shows you how to make it). Just search for those [[ key ]] words. In stock form it's 100 mW, but he supplies high power output transistor and new power supply regulator if you want more output. Final output will be determined by how much voltage you supply to the board.

If you still want to experiment with the Talking house, Google: "Automatic Tuning AM Transmitter". You will find the patent. Some sites let you look at the patent or even down load a jpeg, page by page for free (other wise they want $6 for a pdf copy). If you want the jpeg version, I took time to get it. PM me your email and I'll send it to you. This shows the schematics of the talking house (with out detail of component values). It will be a start in figuring out the design. If you really understand PLL and all the electronics of AM radios, you can have fun with it, at the risk of turning your unit into junk. I do think increasing the output might be more trouble then it is worth, but then you might not feel that way. Good luck and have some Sottish Ale for me at the Ye Olde Pub.

PS
I am more than happy with 100 mW. I have a range of almost 200 feet (strong signal) and can faintly hear it down the street with the stock wire indoor antenna. If I improved my antenna, made a base loaded resonant antenna (with in the 3 meter long limit), with a good ground system (in the earth ground), I could no doubt get about 1/2 mile to 3/4 mile. You know MW AM is very dependant on ground waves and a GOOD earth ground. The better the ground the better the range. You can't make a ground good enough. I am talking about multiple long copper radials in or on the ground (dirt). Moist dirt is a better conductor, which shouldn't be an issue in Scotland.

I finally got around to experimenting with the values of C301 and C337 in my TH 5.0. For those of you who have read my previous post and looked at the audio frequency sweep I did, you probably came to the same conclusion I did.

That is, the high frequency response, while not great, was for the most part adequate for our old AM radios. However, the low frequency (bass response) was noticeably weak, especially so on console style antique radios that do a pretty good job of outputting lots of bass response.

I used the same test setup that I had used in my previous test. I found that C301 does in fact greatly affect the low frequency (bass) response. I tried several different values for C301, but found just as George (GMCJetPilot) did, the best value was about .15uf. Values in the range of .1uf to .15uf were all OK.

Lower values did not give enough low end and higher values gave too much bass boost. In fact I tried values of 1uf, 4.7uf, and 10uf. They all did work, but the bass boost was so high that I had to reduce the input levels to avoid distortion on the low end. It had the effect of reducing the level of midrange and high audio frequencies that could be heard.

For C337, I found that any increase in value above the 47pf that is normally used just decreased the high frequencies. Not acceptable! I did not bother to remove C337 and try a lower value because I deemed the high frequency response was adequate with the factory supplied value for my needs. Others may want to reduce C337 somewhat and see if any improvement is noticed.

So, my final results were as follows:

C301 .15uf - I just tacked a .1uf capacitor across the .047uf cap originally supplied.

C301 .15uf - I just tacked a .1uf capacitor across the .047uf cap originally supplied.

C337 No change - I used the 47pf as originally supplied.

My TH 5.0 sounds pretty good now.

Ed

Thanks Ed. I am glad you got the same results, however I have to
mention two other folks found they could take higher values for
C301.

I DID remove C337 and found improved highs with a lower value. I
did not have an extensive selection of puff capacitors, but I agree
stock 47 pF is fine. However I did go down a little and noticed
improved highs. Adding capacitance over 47 pF does reduced
highs. I like highs. Reduced from 47 pF a tad, I can hear it. I would
like to experiment some more but need some smaller capacitors to
swap in. Right now I have a 39 pF (+/- 5 pF) in there. My capacitor
meter does not measure small capacitors with extreme accuracy,
so I don't know what I actually have, but it's less than 47 pF and
sounds better to me.

Again the old Talking House variability from unit to unit seems to
be in play. Also it also may be affected by what you feed the unit.
If you do experiment try and have flat EQ and no compression or
limiters..... You should size to match what kind of audio you use.

I have found after this Mod, the joy of broadcast software audio
enhancement... There are some free ones and for fee ones. The
problem with the best ones is they use massive CPU. I can run
stereotools with a 2.8 Mhz P4, 768Mb ram, with out breathing
too hard, with some reduced settings. On my 1.3 Mhz P4, 512Mb
ram computer stereotools uses a big chunk of CPU. It works but
you can't do much else with the computer. The software is doing
real time digital audio processing. The commercial software says it
needs duo core processors, although I have heard they will work
on fast P4's with a Gig or two of ram. Even if you don't use the
software, even something like WinAmp's EQ and software that
comes with the better sound cards can improve your broadcast
sound.

Making the one or two cap changes and feeding the TH with a
computer running the software (limiter, compression, EQ with
selective gain, loudness limits....) the audio is a joy.

I have tomention two other folks found they could take higher values forC301.

Yes, I think it is a matter of how you set the input level. I could use values of 1uf up to 4.7uf. They provided so much boost of the low end at about 100hz that I had to reduce the input level to eliminate the overmodulation and distortion at that frequency. Once I reduced the input level, the transmitter sounded fine. The only problem was that the modulation levels were not flat. Under this setup, the mid and higher frequencies had reduced modulation levels. I was striving to maintain as flat as possible of a response from 50hz to 8khz. But, if someone was just listening by ear and not using a signal generator and an o'scope, they would only be able offer an opinion as to what sounded good to them and each person's opinion would differ as to what values of C301 sounded the best.

Quote:

I DID remove C337 and found improved highs with a lower value.

When I get some time, I will clip out C337 and try some lower values to see if the high end can be improved on my unit.

When I get some time, I will clip out C337 and try some lower values to see if the high end can be improved on my unit.

I spent a little more time on my TH 5.0 yesterday. I started by clipping one lead of C337. I then did a frequency sweep. I found that with C301 set at .15uf and C337 removed the overall response was pretty flat from 50hz to 15khz. Not bad!

I tried several other values for C337 from 10pf to the original 47pf and all of them decreased the high end response. So, I found it better to just leave C337 disconnected. Others may have different results.

I then did a comparison between the TH 5.0 and the SSTRAN. I found that I still like the sound quality of the SSTRAN a little better. However, the TH 5.0 is much better now than what it was before I started and I would consider it pretty decent now.

Whoever it was that was able to figure out that the audio response determining components on the TH 5.0 are C301 and C337 was correct. Those are definitely the two values to adjust.

Whoever it was that was able to figure out that the audio response determining components on the TH 5.0 are C301 and C337 was correct. Those are definitely the two values to adjust.Regards, Ed

Glad it worked out for you. You are welcome! Please send me a
check. Thanks for confirming it. I am glad yours is like mine.
I was wondering based on the other ARF member. I can only guess
that there is some fundamental difference with in the production run.
I was assured by Talking House (now Radio Systems) that they never
changed the design but that it's likely substitutions were made and
component quality may have varied.

For some reason to my ear dropping C337 from ceramic 47 pf (who
knows actual value, tolerance) to 39 pf (+/- 5%), a high quality mica
(all I had), made the highs a little more sparkly, brighter, clear? It
could be quality of capacitor? Ceramic is a POOR choice.

Additional Mod to consider:

Speaking of poor choices there are ceramics in the PLL circuit, which
is also powered off the RF rail. This is poor design, it was pointed out
to me. I noticed that commercial FM stations sounded better on SSB.
If you listen to the Talking House on SSB you will hear the wobble, or
what some call "FM'ing". The Freq varies slightly. The PLL is a nice
feature the TH has but implemented some what poorly. By replacing
caps and providing dedicated power supply, the stability will improve.
This FM'ing is not noticeable on an AM radio but SSB it is. It's just
poor form and not going to contribute to best sound. Mine does it a
little some times. It varies at times.

An error in one of the photos..first image with "G" as output transistors...incorrect. Those are the driver transistors for the motor that drives the internal ATU. They serve as a PWM circuit so that the motor at any applied voltage maintains torque as it adjusts the tuning unit.

The audio improvements made to date do help, but do not solve the source of the audio problem. I have recently purchased a TH 5.0 unit. I have to inject here that until the FM'ing is eliminated, the audio will never be right.

As I noted in another forum, I will begin work on eliminating this FM'ing issue and post my results there as well as here. It is an isolation issue between the audio modulation circuits and the main oscillator.

An error in one of the photos..first image with "G" as output transistors...incorrect. Those are the driver transistors for the motor that drives the internal ATU. They serve as a PWM circuit so that the motor at any applied voltage maintains torque as it adjusts the tuning unit.

Yes I put a circle around that area and the letter G. Yep my mistake what was I thinking! Those two big transistors near the autotune antenna is not the RF output but PWM control of the motor for autotune antenna. Which one is RF output? Do you know?

Quote:

The audio improvements made to date do help, but do not solve the source of the audio problem. I have recently purchased a TH 5.0 unit. I have to inject here that until the FM'ing is eliminated, the audio will never be right.

Yes I mentioned FM'ing above. However "will never be right" may be technically true, it's not that big of a deal. Most people will not notice it or hear it. It is "bad form" to have some "Freq wobble" but it's better than non PLL transmitters, but for a PLL transmitter it's not ideal, granted. Does it need to be fixed? I am done with the TH mods. It sounds great for what I paid for mine (basically free). For $40-$60 typical used price they are a bargain for a turn key digital tune transmitter. The SSTran may sound a little better, but with the TH audio mod the difference is small. Also the SSTran has crummy range, more than OK for the house, just not as much as the TH. The SSTran is a kit as well which may be a pro or con depending on the person. I think the TH has WAY too many parts in it. That whole autotune antenna section is a big part of it. It's more complicated than needed, but it works. It might be a bit of a pain to mod, so I am starting from scratch.

I am building a C-QUAM AM transmitter. It is going in a WX proof box in the garden, with a tuned mid loaded 9-foot long mono-poll and a TON of ground radials. I expect up to 1/2 mile range of clean AM. Of course AM stereo plays on mono receivers. It is a Freq transmitter with replaceable programmable crystals, which are a fraction of the price of regular crystals. Mouser or Dig-Key can program it for you when you order it. Since I am tuning to one Freq and the antenna is tuned externally, the transmitter is much more simple.

Quote:

As I noted in another forum, I will begin work on eliminating this FM'ing issue and post my results there as well as here. It is an isolation issue between the audio modulation circuits and the main oscillator. As they say....stay tuned! RFB

Let us know. As I mentioned above the power rail for the PLL is on
the same RF power rail (source). Not an ideal design. I also
mention the ceramic. Not the ideal type of capacitors in a PLL circuit.

Some great info here, thanks to all for the input. I will be acquiring a TH 4 very soon so may need to do the audio mods.
I have a question (or 2).... on the 3 meter wire antenna included, what would happen if a length of coax was connected, centre core to terminal and screen to chassis and then the wire antenna at the other end outside and the screen to an earth rod, would the built in ATU still work and do the job or would it be pointless? Also what if a longer wire was used in the same scenario? Just want to experiment with it really, I am in the UK so FCC not a problem, just other forces...lol
On another note, could the channel spacing be changed to 9 kHz from 10 kHz or again is that just pie in the sky?
Thanks.

Some great info here, thanks to all for the input. I will be acquiring a TH 4 very soon so may need to do the audio mods. I have a question (or 2).... on the 3 meter wire antenna included, what would happen if a length of coax was connected, centre core to terminal and screen to chassis and then the wire antenna at the other end outside and the screen to an earth rod, would the built in ATU still work and do the job or would it be pointless?

It would be a possible distortion in the spacetime continuum and end life as we know it. I understand what you want to do. My honest answer is I don't know and don't do it. But HEY try it. The reason I say don't do it is that AUTOTUNE system that feeds the single thumbscrew terminal on the back is looking for a wire of a specific impedance and resonance (inductive and reactant). IT MAY NOT WORK. If the coaxial is short it will not matter. Short being may be ~15 feet or less? It may hunt and never self tune or tune way off... try it.

I'd use the 'F' connector antenna jack for coaxial. You have to throw the switch on the back of the TH to outdoor antenna. This disables the audio tune circuit. You have to manually match the antenna.

1) The most simple, run your coaxial from the F output to your antenna. Then tie the center and shield with a 50 ohm resistor; extend your 3 meter wire from there; no ground. It's not great for range, but it will assure proper impedance. No ground needed. This is SUPER EASY, WORST PERFORMANCE. It however may be quiet if you got the antenna outside. Remove the resistor and see what happens.

2)Wind a loading coil (about 62 turns on a 4" PVC) and three meters of wire/copper/aluminum pipe/tube. Since you live in UK, and have room for 150' of wire you can skip the loading coil. Coaxial shield goes to the ground stake/ground radials/ The more ground radials and the longer the better, 15'-20' each or what you have room for. Tap the loading coil every other turn starting about half way. For an impedance matching coil (link). Plan on using higher Freqs. The lower the freq the longer the antenna needs to be.

Quote:

Also what if a longer wire was used in the same scenario? Just want to experiment with it really, I am in the UK so FCC not a problem, just other forces...lol On another note, could the channel spacing be changed to 9 kHz from 10 kHz or again is that just pie in the sky? Thanks.

Go for it, the longer the better. I am not sure how much the AUTO TUNE can deal with. If it tunes it, then go as long as you can. If yhr autotune does not lock on you get a error message. In your coaxial scenario above, again use the F connector and forgo the tuning coil and run ~150 of wire if you can, along with the ground.

For the autotune antenna a few feet would be goodness, say 14 feet would be OK. For you may be 20 feet, 30 feet would be awesome. If your antenna tuner works it's good. It will help range and audio quality. The manual gives dire warning, but this is a FCC thing. However you do it at your own risk there in Kingdom.

As far as 9 Khz spacing? No fix. You will get lucky every 90 Khz. There is no mod. I looked into it, and smarter people can't see a way to do it. If you have an analog AM (MW) radio just manual tune it to match the Talking House. You may want the SStran transmitter, which has 9 Khz spacing I think.

Wow! Some great advice there, thank you. Regarding the 10 to 9 kHz, I thought as much but needed someone much more clever than me to confirm it. I will do some experimenting with long wire and the ATU, like you say try and see, it really is just experimenting and it will always be at 100mw so I cannot see it bothering anyone here. The TH is on the way to me but once I have it and try different scenarios I will post back results, even if it's just so you can say "told you so"... I suppose you are right with the internal trimmer, if it can't tune whatever wire, coax etc.. then it won't lock so that should answer most my questions. I am actually looking forward to fixing the audio also, assuming it is flat, being a TH 4 it will be from what I have read. I went for one of these since I like the simplicity and the design and they are cheap in the US, this one is new and was well under half of the usual $99, even with shipping and possible taxes (not sure if there will be any since the value is low) it is still cheap so worth a play and FM has served it's purpose to me, time to try something different so this site and particularly this thread has been a wonderful insight.

you say try and see, it really is just experimenting and it will always be at 100mw so I cannot see it bothering anyone here. The TH is on the way to me....

The 4 and 5 are basically the same. One thing that TH is famous for is variation between units, even with in the same model number, 4.0, 5.0. Some say theirs sounded fine out the box, others not so much. Either way you can make it sound great.

Also I and others have found that the TH are making way more than 100 mw... at least 140 mw at the higher Freqs and doubt that at the lower Freq. Like I said the antenna becomes less efficient at lower freqs because it's way too short. Even if you make a loading coil it's less efficient. So the extra power is not really that useful at the lower end. However the TH has better range than the SStran which is a flat 100 mw... How they go FCC certification I don't know. The rules say 100 mw at the antenna. So if you do use a coxial there will be loss. In that case you do want more than 100 mw to make up for the loss to get the full 100 mw to the antenna. It is all academic for you in UK.

In the USA the MW band is crowded... very. So finding good Freqs that are open day and night is most important. Not sure of the situation on the Island. In the USA the higher Freqs 1500-1710 tends to be less crowded and are less noisy.

Audio input is critical for good sound. If you are just a little computer literate (I am sure you are and it does not take much skill). Get Winamp (free audio player). Then get the free plug in called "Stereo Tools". It is a fancy multi band EQ, limiter, compressor and filters that can do wonders. Play you MP3 or internet radio (Winamp has shoutcast which is internet radio). Stereo Tools has presets for FM mono radio setting and AM mono radio settings work well, or you can customize it as you like. It's free so try it. This with the capacitor mod is goodness. My "station" sounds as good as the best local commercial AM stations.

Hum seems to plague some users. That is noise in the house but there are many causes. Search the ARF archives or start and thread if you have problems with HUM... often just moving the antenna or receiver fixes it.

You will enjoy it. Some people complain about audio quality, others are happy with the TH as is. Again the variation between models is suspected. The gouge on how to make it sound good should help you get max enjoyment. The mod makes noticeable improvement on any model.

BTW Some where I recall reading UK/BBC was ending ALL MW services? How many stations are in Britain. I assume you get stations across the channel.

The MW band here is not too busy, we have local BBC stations still but they will be phased our, BBC 5 live, not sure of it's future on AM, Talk Sport, Absolute Radio which used to be Virgin radio and is up for sale again, the MW future for that is unknown and there is a national Gold station but different areas are being shut down so it will be quieter in the future from the UK anyway. Here is a link to the list if you do want to know: List of UK AM stations A lot of those are local stations.
I would only be looking at the higher frequencies, 1440 and above probably. I have done internet radio and had FM transmitters so am up to speed with winamp, stereo tools etc.. This AM idea is just different, that's what appeals to me although making antenna's looks a lot harder than making them for the FM broadcast band, a dipole for that is so easy, not quite the same for AM! Some great info here and around the net for antenna's though so that's a big help.

that's what appeals to me although making antenna's looks a lot harder than making them for the FM broadcast band, a dipole for that is so easy, not quite the same for AM! Some great info here and around the net for antenna's though so that's a big help.

Yep AM is harder to get to sound good. HOWEVER you can get a full 5 Khz of baseband audio, with lower response above to 8 Khz (at least in the USA). That may sound crummy compared to FM stereo of 15 Khz audio, but there is not a lot of ACTUAL musical notes or singing above 4 Khz... of course the area from 8-15 Khz is called the BRILLIANCE freqs. They do add the sizzle to the sound.

That is why in USA AM has been relegated mostly to TALK, SPORTS, ETHNIC SPANISH and OLDIES (where music was not recorded in "Hi-Fi"). FM became the de facto music band in the 70's. However with a GOOD AM receiver and good AM transmitter and the audio just right, the sound is very pleasant. Oldies and old blues and folk music old mountain blue grass, like that song you heard in the George Cloney movie. Here are the Soggy Bottom Boys - "Man of Constant Sorrow":

Obviously this song is a modern recording. Also when you get older, most men start losing their high freq audio hearing any way. I know some old guys who can't hear the difference between AM and FM. In the USA we have 50,000 watt MW BCB stations that are "Clear Channel". That Freq is protected. At night any on that Freq have to drop power and or go directional. Thus thus these 50KW stations can transmit +500 miles with little effort at night.

I grew up listening to AM radio anyway in the 80's except for on a Sunday when BBC Radio 1 had the UK top 40 on the Radio 2 FM frequency for 2 hours, that was a luxury compared to AM but I do obviously have a love for good ol' MW. I am looking forward to playing with the TH, probably have to wait a week or more still for it to arrive but will post back when it does.